Optimizing Slurry Seal Mixtures with Different Aggregate Types: Performance, Durability, and Component Interaction Analysis

Abstract

The primary objective of this study is to understand the aggregate–emulsion interaction in slurry seal coatings and to obtain cost-effective, improved performance characteristics by overcoming the restrictive effects of aggregate chemistry on workability through hybrid designs. In this study, the interactions of granite (GR), basalt (BA), and limestone (LS) aggregates with bitumen emulsion were examined; specifically, limestone-substituted designs were analyzed to overcome workability problems stemming from the high reactivity of basalt and to achieve optimum performance. Laboratory specimens were subjected to mixing time, cohesion, Wet Track Abrasion (WTAT), and Loaded Wheel (LWT) tests in accordance with the procedures specified by the International Slurry Surfacing Association (ISSA); and the effect of the determined optimum emulsion content on performance was analyzed with ±2% sensitivity. While experimental findings indicated that the predicted optimum emulsion contents for all selected aggregate types satisfied the specification limits, the mixture with 30% basalt substitution (LS70+BA30) among the hybrid designs achieved the highest design compatibility by providing an exact match between theoretical and experimental optimum points. Conversely, despite having lower design sensitivity (66.7% match), the mixture with 50% basalt substitution (LS50+BA50) offered a superior alternative for situations requiring quick opening to traffic by exhibiting 54% higher early cohesion strength (20 kg-cm) at 120 min compared to pure limestone. Statistical analyses confirmed that aggregate origin is the most determinant factor on mixing time and that the fluidity characteristic of the system is predominantly controlled by water content. Furthermore, correlation matrices demonstrated the need to optimize the liquid phase balance in hybrid designs according to aggregate mineralogy by revealing the rheological sensitivity developed by limestone towards water and granite towards emulsion. Through the study outputs, the restrictive effects of aggregate chemical content on workability—which have not been sufficiently detailed in the literature compared to the frequently discussed effects of particle size distribution and mineral filler in slurry seal mixtures—were identified, and solution strategies were developed.

1. Introduction

Asphalt is the most preferred pavement material worldwide due to its low cost, rapid application, easy repair, flexible structure, and driving comfort. However, it is not a sustainable option for nations considering its negative environmental impacts and the rapid depletion of limited resources. Therefore, in recent years, interest in preventive asphalt applications offering eco-friendly and economic advantages has increased. In road pavements, applying preventive maintenance before damage caused by environmental effects and heavy traffic loads reaches a critical level can delay the deterioration process and eliminate the need for comprehensive maintenance [1,2,3].

Slurry seal, one of the preventive maintenance alternatives, was first applied in Germany in the 1920s and gained significant attention starting from the 1960s with the introduction of advanced emulsifiers and continuous flow machines [4]. This material is a mixture consisting of emulsified asphalt, mineral aggregate, water, and additives [5].

When slurry seal is used as a preventive maintenance application on asphalt pavements, it is stated that its expected service life ranges between 5 and 10 years [4]. However, this duration may vary depending on factors such as the durability of the protective layer, traffic density, environmental conditions, and the condition of the underlying pavement [1,6,7,8]. Although some sources state that the main deterioration mechanism of this process is surface oxidation and abrasion [9,10], the compatibility of the materials selected during the mix design is also highly influential [11].

Compared to traditional hot mix asphalt mixtures, the design of micro-surfacing mixtures such as slurry seal can be more complex. The reason for this is that these mixtures rely on a chemical system consisting of many variables, such as the type, composition, and modification of the asphalt emulsion, aggregate types and gradation, and amounts of water and filler. These variables, which are suggested to directly affect the properties of the final asphalt mixtures [11,12], are frequently addressed in studies conducted by researchers in the literature. These factors, which draw the most attention in studies and affect the performance of micro-surfacing surface properties, are presented by being categorized under three main headings.

1.1. Effects of Filler Materials in Slurry Seals

In slurry seal mixtures, the ratios of aggregate, bitumen, and air voids are approximately 80%, 15%, and 5%, respectively. Since bitumen exists as a very thin film with a thickness of only 10–25 microns in this system [13], when introduced into the mixture, it first forms a “mastic” structure by integrating with the filler material (mineral particles with particle sizes smaller than 0.075 mm). This structure primarily coats the aggregates and ensures their bonding to each other [14]. Subsequently, it fills the voids and serves to transfer loads [15]. In terms of stiffness, strength, and cohesion, the mastic content in the mixture significantly affects the performance of the asphalt mixture, its resistance to rutting, cracking, and moisture damage [16]. Therefore, there has been a considerable number of researchers investigating the significant role of this interaction between filler and bitumen in enhancing mixture performance and quality for many years.

Recent research has focused on the use of various recycled waste materials as alternatives to traditional mineral fillers [13,17,18,19,20,21,22], aiming to develop eco-friendly and high-performance asphalt mixtures. The common objective of the studies presented in

Table 1. Recent waste materials used in slurry seal mixtures and their effects.

Material	Advantages	Disadvantages
EAF steel slag [17]	Curing and mix performance	Limited use, high binder demand & logistics
Waste glass powder [18]	Abrasion, stripping & rut resistance	Limited use & workability issues
Ceramic waste powder [19]	Cohesion, abrasion & stability	Inhomogeneity, rapid stiffening & limited use
Reclaimed Asphalt [21,22,23]	Abrasion & friction performance & cost reduction	Extended curing & additional compaction
Fly ash [20]	Synergistic performance with cement	Poor emulsion compatibility & bonding loss
Recycled tires [21]	Rut, fatigue & noise resistance	Binder demand & stability issues
Fiber [3]	Cracking resistance	Moisture sensitivity

is to enhance fundamental performance criteria—such as adhesion, abrasion resistance, and deformation resistance—while simultaneously ensuring long-term durability and sustainability. However, it has also been noted that the use of these materials entails certain disadvantages.

Studies examining the effects of different mineral fillers (Portland cement, hydrated lime, and quicklime) on the set-time and cure-time of mixtures have stated that increasing the filler content decreases the consistency of the mixture and, accordingly, increases the water demand. Furthermore, it has been highlighted that the use of 2% Ordinary Portland Cement reduces the setting time to under 5 min and provides optimum hardening with rapid-setting emulsion (CQS-1 h) [24]. Pittenger and Gransberg [2] demonstrated, based on field data obtained from a road section over 36 months, that Portland cement-based slurry seal and microsurfacing are economically viable solutions with similar trends for filling rutting on road surfaces. In a similar study, two different slurry seal mixtures were tested: one containing cement as filler and virgin aggregate, and another containing hydrated lime as filler. The results pointed out that while low-cost, acceptable performance improvements were achieved, additional measures (such as rolling to improve friction life) might also be required [22]. Performance analyses from a study on the use of basalt filler emphasized the positive effect of inert basalt on hardness; it was reported that, albeit rarely, rutting and friction resistance improved when combined with hydrated lime [13]. Additionally, the study by Oikonomou and Eskioglou [25] stated that alternative fillers such as fly ash, steel furnace slag, cement kiln dust, and marble powder met ASTM and ISSA criteria in performance tests, including set time, consistency, and wet track abrasion. These results indicate that selecting the appropriate filler type and utilization rate not only maintains traditional performance but also provides advantages in terms of environmental sustainability.

In slurry seal and similar systems used as preventive surface treatments in road pavements, the type and amount of filler included in the mixture constitute one of the fundamental control mechanisms determining not only the mechanical strength of the material [19,26], but also the workability and long-term performance of the mixture [27].

Research has shown that adhesion performance between asphalt and aggregate can be increased by improving filler properties and using alternative materials to meet regulatory requirements and even achieve improvements exceeding target limits. Studies conducted for this purpose shed light on the development of innovative solutions in road technology by reducing the risk of limited resources.

1.2. Effect of Bitumen Emulsion in Slurry Seals

Gransberg [28] emphasized that the primary design objective of thin protective coatings is to ensure compatibility between mixture components and to meet strength criteria. In this context, a strong chemical bond between the aggregate and bitumen emulsion is considered particularly important in mixtures [29]. Asphalt emulsion consists of asphalt cement (AC), water, and an emulsifier (surfactant). Generally, asphalt cement constitutes approximately 60% to 70% of the total volume of the asphalt emulsion. Durability and early coating adhesion can be improved in asphalt emulsions through the use of polymer-modified asphalt cement (AC) and latex, as well as by selecting cationic, anionic, or nonionic emulsifiers that enable asphalt cement particles to form small suspended droplets in de-ionized water [11,30]. These parameters affect not only the structure of the emulsion but also its interaction with the aggregate surface because the chemical composition of the emulsion is one of the fundamental factors determining its adhesion capability to the aggregate and the overall integrity of the mixture.

In a study conducted by Zalnezhad and Hesami [31], the performance of microsurfacing mixtures using different emulsion types and steel slag aggregates was investigated. In the study, six different microsurfacing mixtures were designed using two different emulsions (slow-setting and rapid-setting cationic emulsions) and varying aggregate compositions: 100% silica aggregate, 61% steel slag aggregate, and 100% steel slag aggregate. ISSA A143 [32] and ASTM D6372 standards [33] were used for the preparation and testing procedures of the mixtures. The performance tests applied to the mixtures included the cohesion test, Wet Track Abrasion Test (WTAT), and Loaded Wheel Test (LWT). The results of the study indicated that due to the alkaline nature of steel slag, it provided better adhesion with acidic bitumen emulsion, thereby yielding better experimental results compared to the use of silica aggregate alone.

Hu et al. [34] investigated the effects of using waterborne epoxy resin-modified bitumen emulsion in preventive maintenance applications for road pavements. In the study, fluorescence tests were conducted to examine the distribution of the additive used within the bitumen matrix, while Wet Track Abrasion, skid resistance, and friction tests were applied for performance analysis. The test results revealed that the waterborne epoxy was distributed uniformly within the bitumen matrix and that the modified bitumen emulsion significantly improved the skid resistance and durability of the coatings.

1.3. Effects of Bitumen Emulsion–Aggregate Bond in Slurry Seals

Each component of the structure formed by the combination of aggregate and bitumen emulsion possesses a distinct chemical composition, and this composition directly affects the bond between the aggregate and the bitumen emulsion. The bitumen emulsion used in micro-surfacing is generally cationic. The use of aggregates with a surface charge similar to cationic bitumen, such as limestone, leads to weak adhesion between bitumen and aggregate, and also delays the setting rate and curing time of the mixture [31,35,36].

In a study conducted by Liu et al. [37], the adhesion between emulsified asphalt residue and aggregate was investigated using molecular dynamics simulation, and it was stated that low emulsifier content (1–3%) forms a stronger interfacial bond. Furthermore, it was found to be more resistant to temperature and moisture due to its molecular arrangement being more similar to that of base asphalt. Additionally, it has been reported that the presence of silica–water and asphalt–water hydrogen bonds contributes to increased resistance against moisture-induced damage [37].

The adhesion capability at the interface between emulsified asphalt residue and aggregate is considered by researchers as the weakest link in asphalt mixtures [38,39]. When proper bonding is not achieved, stripping and separation occur at the interface; which reduces the mechanical properties of the mixtures and degrades their service performance [40,41,42].

In another study highlighting the importance of binder–aggregate interaction on performance, ref. [43] stated that adhesion can be reliably measured with modified sweep, Vialit, and pull-off tests applicable in field conditions; and also demonstrated that aggregate surface conditions (dry, wet, dusty, clean) directly affect binder retention.

1.4. Effects of Aggregate Chemical Composition on Slurry Seal Mix Design and Performance

The chemical composition of aggregates plays a critical role in determining the breaking behavior, cohesion development, and general performance of slurry seal mixtures. The chemical and mineralogical properties of aggregates determine parameters such as surface charge, acidic/basic character, and reactivity. These properties directly affect the interaction with cationic bitumen emulsions. When aggregates come into contact with water, they acquire a characteristic surface charge depending on the mineral type and pH value. Silica-rich (acidic) aggregates, such as granite and basalt, generally carry a negative surface charge, while calcium carbonate-rich (basic) aggregates, such as limestone, possess a positive surface charge [44,45]. As the pH value increases, the surface of most aggregates becomes more negative, which reduces the attraction force with cationic emulsions. In contrast, divalent cations such as ${\mathrm{Ca}}^{2+}$ and ${\mathrm{Mg}}^{2+}$ found in basic aggregates neutralize the surface charge, promoting faster breaking and adhesion development [23]. Therefore, a thorough understanding of the chemical reactivity and polarity of the aggregate surface is of critical importance for predicting breaking time, cohesion, and performance in slurry seal mixtures.

Although basalt and granite are classified as silica-based and acidic aggregates, they show significant differences in terms of oxide compositions and surface reactivities, and these differences directly affect their interactions with cationic bitumen emulsions. Since granite generally contains 65–75% ${\mathrm{SiO}}_2$, it presents a more inert and chemically stable surface. This property reduces electrostatic attraction with positively charged emulsion droplets and results in slower breaking behavior [46,47]. Conversely, basalt aggregate contains high rates of ${\mathrm{Fe}}_2{\mathrm{O}}_3$, $\mathrm{MgO}$, $\mathrm{CaO}$, and ${\mathrm{Al}}_2{\mathrm{O}}_3$. These oxides form local basic regions when in contact with water. Ion exchange reactions occurring in these regions cause the pH to rise, accelerating the early coalescence of bitumen droplets [45]. Therefore, mixtures containing basalt generally tend to break faster and have shorter workability times compared to mixtures using granite or limestone.

Figure 1 illustrates the normalized geochemical compositions (left) and application properties (right) of the basalt, granite, and limestone aggregates used in this study. The radar chart clearly reveals the high ${\mathrm{Fe}}_2{\mathrm{O}}_3$-MgO-CaO content of basalt; the dominant ${\mathrm{SiO}}_2$-${\mathrm{K}}_2\mathrm{O}$-${\mathrm{Na}}_2\mathrm{O}$ composition of granite; and the high CaO ratio of limestone. The reactivity-breaking speed diagram on the right of the figure shows that basalt exhibits high chemical reactivity and rapid breaking; granite exhibits moderate reactivity and medium breaking; and limestone exhibits low reactivity and slow breaking characteristics.

Similar trends have also been reported in the literature [48], which demonstrated that the ${\mathrm{CaCO}}_3/{\mathrm{SiO}}_2$ ratio is the primary factor determining the interaction energy at the emulsion–aggregate interface and that basic oxides accelerate emulsion breaking. Husain et al. [23] stated that aggregate surface charge and oxide composition significantly affect the cohesion development of emulsion–aggregate mixtures, and specifically, that MgO content increases the early curing tendency. Furthermore, it was emphasized that polar interactions between asphalt molecules and mineral surfaces are decisive in the adhesion and breaking mechanisms in emulsion systems.

Consequently, the difference between Fe-Mg-Ca-rich basalt and Si-Al-alkali oxide-rich granite has a significant impact on the chemical reaction and performance of slurry seal mixtures. Therefore, the combined use of aggregates with different chemical reactivities in specific proportions offers an optimum design approach in terms of the reactivity-breaking time-workability balance [23,44,45,47,48].

Additionally, recent multi-scale studies conducted by Jiang et al. [49] in 2025 have further clarified the relationship between the chemical composition of aggregates and bitumen interface adhesion. The researchers established a performance hierarchy regarding adhesion quality as limestone > basalt > granite. Specifically, they confirmed that calcium-rich ($\mathrm{CaO}$) and mafic (${\mathrm{Fe}}_2{\mathrm{O}}_3$, $\mathrm{MgO}$) mineral structures exhibit a stronger ionic bonding potential in aqueous environments compared to silica-rich (${\mathrm{SiO}}_2$) surfaces.

In the literature, the comparisons presented in

Table 2. Comparison of the physical, chemical, and interfacial properties of aggregates with different lithologies.

Aggregate Property	Limestone	Granite	Basalt
Origin [50]	Sedimentary	Magmatic (Int.)	Magmatic (Ext.)
Chemical Character [44,45,46]	Basic	Acidic	Neutral
Particle Shape [46,51]	Sub-angular	Angular	Highly angular
Roughness [13,46,52]	Low	Medium	High
Porosity [46,49]	Micro	Low	Micro-Macro
Surface Reactivity [13,23]	Low	Low	High
Surface Charge [44,45]	(+)/Weakly (−)	(−) (Inert)	pH-dependent
Source of Surface Charge [44,45]	${\mathrm{Ca}}^{2+}$	$\mathrm{Si}\text{-}{\mathrm{O}}^{-}$	${\mathrm{Fe}}^{2+}$, ${\mathrm{Mg}}^{2+}$, ${\mathrm{Ca}}^{2+}$
Cationic Emulsion Interaction [31]	Slow-controlled	Slow-weak	Fast-strong
Breaking Speed [13]	Slow	Slow	Fast
Effect on Workability [49]	Increases	Neutral	Decreases
Adhesion [52]	Medium	Medium	High

, which illustrate the different lithological properties of aggregate types used in slurry seal mixtures, are of critical importance. These characteristics serve as key factors in understanding the adhesion quality and interaction mechanisms within the resulting mixtures.

1.5. Research Significance

This study investigates the effects of different aggregate chemical compositions and mineralogical properties on the design parameters and performance behaviors of slurry seal mixtures. In the analyses, three different types of aggregates—limestone, granite, and basalt—were used, each characterized by distinct origins, chemical properties, morphological structures, and electrical charges, resulting in different mechanisms of interaction with the emulsion, as specified in Table 2. Among these aggregate types, basalt aggregate, due to its high Fe, Mg, and Ca oxide content, exhibited high chemical reactivity when combined with the cationic emulsion. This led to excessively rapid breaking of the emulsion, preventing the mixture from meeting the minimum mixing time and consistency determination criteria specified in the ISSA A105 [5] standards.

Although there are various studies in the literature examining the effect of aggregate gradation and filler properties on slurry seal performance [19,53,54], the effect of aggregate chemical composition on emulsion breaking speed and optimum emulsion content has not been sufficiently investigated. In particular, systematic research on the use of highly reactive aggregates such as basalt in slurry seal systems and the implementation of aggregate blending strategies to ensure the reactivity–workability balance is limited [45,46,51,55].

In order to address these knowledge gaps in the literature and ensure the applicability of basalt aggregate under slurry seal conditions, basalt–limestone and basalt–granite mixtures were prepared in different proportions. This experimental approach aimed to achieve a controlled breaking speed and appropriate workability; thereby, the optimum mixture composition meeting ISSA A105 [5] performance standards was determined. Within the scope of the study, gradation and methylene blue tests were conducted in the laboratory environment to evaluate the physical properties of the aggregates. The chemical compositions of the aggregates were evaluated by taking into account XRF data reported in the literature. A cationic slow-setting bitumen emulsion containing SBR latex and Polyram L927 emulsifier was used in the mixtures; and performance evaluations were conducted via consistency determination, mixing time, wet cohesion, Wet Track Abrasion (WTAT), and Loaded Wheel (LWT) tests. When the results obtained from the analyses are evaluated together with the literature, it can be stated that aggregates with different chemical compositions may exhibit highly variable and sensitive interaction potentials with complex materials such as bitumen emulsions, which consist of two distinct phases (bitumen phase and water phase). This sensitivity was experienced in the laboratory tests conducted within the scope of this study and was clarified through the evaluation of the results, particularly in terms of determining optimum mixture components and assessing performance in slurry seal mixture production, where significant difficulties may arise. According to the study results, the use of granite in combination with basalt was insufficient to overcome the workability problems associated with basalt. However, owing to the stable and superior adhesion mechanism provided by the calcium carbonate ${\mathrm{CaCO}}_3$ structure of limestone, the most striking outcome of the study is the recommendation of a hybrid design consisting of 70% limestone and 30% basalt. This new proposal is considered to represent a clearly defined aggregate proportion distribution that offers an innovative contribution to the literature. In addition, experimental findings were obtained that are fully consistent with the theoretically determined optimum component contents. When these results are evaluated together with the statistical analyses, they provide valuable insight into the development of applicable strategies for addressing workability problems encountered in engineering applications.

2. Materials and Methods

2.1. Materials

2.1.1. Aggregate

The diversity of aggregate types utilized in the slurry seal specimens prepared within the scope of this study is of significant importance. Three distinct aggregate types were employed in the prepared slurry seal specimens: limestone, basalt, and granite. The limestone aggregate was obtained from the Habibler Region of Istanbul Metropolitan Municipality Asphalt Factories (ISFALT); the basalt aggregate was procured from the Kırklareli-Vize Region Facility of Çalışkan Mining; and the granite aggregate was sourced from the Çorlu-Karatepe Region Facility, also owned by Çalışkan Mining. These materials are classified as stone dust with a 0–5 mm fraction.

Sieve analysis and methylene blue tests were conducted on the aggregates used in the study. The results regarding the aggregate gradations obtained from the sieve analysis are presented in Figure 2. Since the initial aggregate gradations did not fall within the limits of the Slurry Seal Type 1 specification [56], the material was re-sieved in the laboratory using sieves of appropriate diameters to match the average specification values

Table 3. Conformity of gradations of mixtures prepared using different aggregate types to ISSA A105 specifications.

Sieve Size (mm)	Aggregate Sieve Analysis Results (% Passing)			Grading Range (% Passing) for Type I mix	Avg. Values in Mixtures
	Limestone	Granite	Basalt
4.75 (No. 4)	96.8	91.7	99.0	100	100
2.36 (No. 8)	65.6	58.9	74.4	90–100	95
1.18 (No. 16)	42.6	40.2	46.1	65–90	77.5
0.600 (No. 30)	29.0	28.1	28.5	40–60	50
0.300 (No. 50)	22.7	20.9	19.5	25–42	33.5
0.150 (No. 100)	16.5	15.5	13.4	15–30	22.5
0.075 (No. 200)	13.7	11.1	9.7	10–20	15

. The aggregates used in specimen production consist of washed and clean materials.

The methylene blue test [57] was conducted to determine the content of deleterious clay minerals present in the aggregates. The test results are presented in Figure 3. Among the aggregates used in the slurry seal mixtures, basalt was identified as the material exhibiting the highest methylene blue value.

2.1.2. Bitumen Emulsion

Within the scope of this study, a slow-setting latex-modified bitumen emulsion was produced for use in slurry seal mixtures. The emulsion possesses a structure consisting of an oil phase (bitumen) and an aqueous phase (a liquid mixture of water, acid, emulsifier, and latex). The materials utilized in the production included the slow-setting emulsifier Polyram L927 from Ceca Arkema Group (La Garenne-Colombes, France), the latex additive Butonal NX 4190 from BTC Kimya (Kocaeli, Türkiye), and hydrochloric acid obtained from ZAG Kimya (Istanbul, Türkiye) to adjust the pH using water at 40 °C. The emulsion production was carried out in a laboratory-type colloidal mill according to a prepared mix formulation. The stages of the production process are illustrated in Figure 4.

The polymer-modified bitumen emulsion synthesized in this study consists of 62.00% by weight bituminous binder (50–70 penetration grade) and 38.00% soap phase (modified aqueous phase). The water content, constituting the solvent phase of the system, was determined as 32.80% of the total mixture. The ionic balance of the soap phase was stabilized using hydrochloric acid (HCl) to ensure full compliance with International Slurry Surfacing Association (ISSA) standards.

To improve the rheological and mechanical properties of the formulation, latex copolymer was added to the system at a rate of 3.50% of the total mixture weight. Thanks to this modification, the viscoelastic behavior of the bitumen was improved, and its adhesion capability to the aggregate was enhanced. The colloidal stability of the emulsion was achieved with Polyram L927, added to the formulation as an emulsifier at a concentration of 1.70%. This selected emulsifier minimizes the risk of premature coagulation (breaking) prior to the paving process due to the slow-setting character it imparts to the soap phase.

In the preparation of the soap phase (water + acid + emulsifier + latex) used in experimental studies, 5000 g of water was taken as the reference mass. Into this phase, 533.5 g of latex copolymer and 259.1 g of emulsifier were added in a controlled manner; the acid addition was adjusted to achieve the targeted pH value of 2.0 within ISSA limits.

According to the parameters of the production facility operating in a continuous regime, a mass balance was established for a total system flow rate of 300 L/h. According to the process flow diagram shown in Figure 4, a homogeneous mixture of the components was achieved in the colloidal mill by feeding material at ${150}^{\circ }$ and 186 L/h from the bitumen line, and the liquid mixture at 40 °C and 114 L/h from the prepared soap phase line. Since this homogeneous mixture was at a temperature of approximately 83 °C when discharged from the mill’s outlet chamber, it was left to cool at room temperature for one night. With the dissipation of the foaming occurring during the initial production within this period, the material became ready for use in mixtures. The properties of the emulsion used in the study are presented in

Table 4. Test results for bituminous emulsion and compliance with KGM technical specifications.

Test	Method	Result	KGM Tech. Spec.
Breaking Index (BI)	EN 13075-1	223.6	>170 (class 5)
Efflux Viscosity (4 mm, 40 °C)	EN 12846-1	24 s	5–70 s (class 5)
Binder Residue	EN 1431	61.69%	58–62 (class 6)

.

The bitumen emulsion produced in this study complies with the general descriptive criteria and classes for slow-setting cationic bitumen emulsions, established in accordance with the TS EN 13808 standard, which is aligned with European standards and specified in the Technical Specifications of the Turkish General Directorate of Highways (KGM). Table 4 presents the test results indicating the properties of the bitumen emulsion prepared in this study, along with their compliance with these specifications.

2.1.3. Water

The water to be used in the mixtures must be free from harmful water-soluble salts and contaminants. To this end, water possessing these qualities was procured for the prepared mixtures and utilized within the framework of the conditions specified in the ISSA A105 Slurry Seal Mix Design standard [5].

2.1.4. Cement

The cement used in the production of bituminous slurry mixtures significantly influences, in particular, the setting time and stiffness of the mixtures. The cement utilized in the mixtures produced within the scope of this study is a slow-setting pozzolanic cement of CEM IV/B type and 32.5 N strength class, manufactured by Akçansa. A slow-setting cement was selected to enhance the workability of the material, and the appropriate usage rate was determined through relevant tests.

2.2. Slurry Seal Mix Design

Slurry seal mixtures, consisting of well-graded aggregates, bitumen emulsion, water, and mineral filler (mostly cement), are thin surfacing pavement materials. The primary objective of the slurry seal design procedure is to optimize the mixture to be applied under specific short- and/or long-term environmental and traffic conditions by combining emulsion, aggregate, water, and filler components in optimal proportions to meet the targeted performance criteria [5]. Producing the mixture with the desired properties has a direct decisive effect on the service life and field performance of the pavement.

Environmental conditions stand out as a significant parameter in the design of slurry seal mixtures. For instance, while quick-setting slurry seal mixtures are preferred under high temperature and low relative humidity conditions, the use of slow-setting slurry seal mixtures may be required under low temperature and high relative humidity conditions.

Although a significant increase has been observed in the use of slurry seal and micro-surfacing applications in recent years, it is known that the design and test methods for these mixtures are largely based on empirical approaches independent of field performance. In current practices, the A-105 and A-143 design procedures recommended by the International Slurry Surfacing Association (ISSA) and the D-3910 and D-6372 standards published by the American Society for Testing and Materials (ASTM) are widely used in slurry seal design and testing processes [33,58].

The design process of slurry seal mixtures follows a systematic approach involving iterative trial-and-error steps. The general flow of this process consists of material selection, material quality testing, mix design, experimental performance evaluation, and analysis of results, as summarized in the slurry seal design flowchart presented in Figure 5. In the first stage, the aggregate, bitumen emulsion, water, and cement components to be used in the mixture are determined; subsequently, the quality characteristics of the selected materials are evaluated through tests in accordance with relevant standards.

To determine the proportions of the components constituting the mixture, mixing time and consistency tests are performed, and the aim is to meet the limit values specified in the standards using the trial-and-error method. Based on the optimum emulsion content determined at this stage, additional samples with 2% lower and 2% higher emulsion content are prepared and subjected to performance tests. Within the scope of performance evaluation, Wet Track Abrasion Loss, cohesion, and Loaded Wheel Abrasion Loss tests are applied; the obtained test results are analyzed with the help of graphs generated as a function of emulsion content to determine the optimum mixture parameters.

3. Experimental Results

3.1. Determination of Material Content for Slurry Seal Mixtures (Mixing Time and Consistency)

Slurry seal mixtures are obtained by combining well-graded aggregate, emulsion, water, and cement materials under specific conditions and in specific quantities. The appropriate usage ratios of the components contained in slurry seals are determined by Mixing Time Determination [59] and Consistency Determination [60]. When examining the research in the literature to determine the proportion of components in the mixtures prepared within the scope of the study, it is observed that cement usage, particularly in Type 2 and Type 3 graded slurry seal mixtures, varies depending on the requirement but generally ranges between 0.3% and 2%. The water content ratios found in the literature [61] generally vary between 6% and 10% for Type 2 and Type 3 mixtures as well. However, since the gradation selected in this study is of the Type 1 class and the proportion of fine material is higher compared to others—which increases the mixture’s water demand—preferences were made close to the upper limit of the relevant range.

The selection of component ratios in the mixture may vary depending on the aggregate type. Optimum mixture components are determined using mixing time and consistency tests. Within the scope of the study, in order to evaluate the performance of slurry seal mixtures with different aggregate types, basalt aggregate was also investigated in addition to granite and limestone aggregates, which are widely used in literature and practice. However, in mixtures where basalt aggregate was used alone, it was determined that the mixing time and consistency test results, which are among the design criteria for slurry seal mixtures, could not meet the ISSA specification limits even at the upper limit values of material ratios. Therefore, instead of obtaining a slurry seal mixture by using basalt aggregate alone, binary aggregate combinations were tested where basalt aggregate was used in certain proportions with other aggregate types for each aggregate gradation. In this context, mixtures containing solely limestone, solely granite, basalt + granite, and basalt + limestone were prepared at various proportions and subjected to the relevant tests. The selection of component ratios in the mixture varies depending on the aggregate type. Optimum mixture components are determined utilizing mixing time and consistency tests. The different mixing time and consistency values obtained by varying the component ratios for different aggregate types are presented in Table 2, Table 3 and Table 4. Based on these tables, the selected optimum mixture ratios were obtained as 0.5% cement, 11% water, and 13% emulsion for mixtures prepared with limestone aggregate; whereas for mixtures using granite aggregate, these values were obtained as 2% cement, 10% water, and 14% emulsion.

Different mixing time and consistency values obtained by varying component ratios in the mixtures for different aggregate types are given in Table 2 and Table 3. Utilizing these tables, the selected optimum mixture ratios were obtained as 0.5% cement, 11% water, and 13% emulsion for mixtures prepared with limestone aggregate, whereas for mixtures using granite as aggregate, these values were obtained as 2% cement, 10% water, and 14% emulsion.

Upon examining the experimental data in

Table 5. Mixing time and consistency values of mixtures using only limestone as aggregate.

Sample No	Aggregate Type	Aggregate Amount (g)	Material Content (% of Aggregate)			Mixing Time (s)	Consistency (Flow) Value (cm)
			Emulsion	Water	Cement
ISSA Limits						Min. 180	2 < Flow < 3
LS1	Limestone	500	15	7	0.5	45	0
LS2			20	10	0	$>300$	$>3$
LS3			17.5	10	0.5	$>300$	$>3$
LS4			15	10	0.5	$>300$	1.5
LS5			15	9	0.5	150	0
LS6			15	10	0.25	-	0.5
LS7 *			15	11	0.5	780	2.25
LS8			15	8	0.5	110	-
LS9			15	10	0.5	$>300$	$>3$
LS10			15	11	0	1020	$>3$
LS11			15	11	2	630	$<2$
LS12 **			13	11	0.5	420	2.5
LS13			17	11	0.5	1200	$>3$
LS14			11	11	0.5	150	-

* Samples complying with the ISSA specification; ** Sample corresponding to the optimum component content selected within the scope of the study.

, it is evident that both the LS7 (15% emulsion) and LS12 (13% emulsion) specimens successfully met the minimum mixing time requirement of 180 s stipulated in the ISSA TB-113 standard, as well as the target consistency values. However, increasing the emulsion content from 13% to 15% within a matrix of fixed water (11%) and cement (0.5%) resulted in a disproportionate extension of the mixing time from 420 s to 780 s. This phenomenon is attributed to the additional lubrication effect induced by the increased binder phase between aggregate particles, indicating a prolongation of the system’s rheological stability without any technical necessity. In this regard, considering the principles of engineering economy and sustainability, the LS12 specimen was designated as the optimum mixture design, as it achieves the targeted workability criteria with lower binder consumption, thereby representing the most efficient point in terms of the cost–performance ratio.

Among the 16 experimental trials conducted with granite aggregate, specimens GR5, GR9, and GR10 were identified as compliant with the mixing time and consistency criteria, as shown in

Table 6. Mixing time and consistency values of mixtures using only granite as aggregate.

Sample No	Aggregate Type	Aggregate Amount (g)	Material Content (% of Aggregate)			Mixing Time (s)	Consistency (Flow) Value (cm)
			Emulsion	Water	Cement
ISSA Limits						Min. 180	2 < Flow < 3
GR1	Granite	500	15	11	0.5	360	3.5
GR2			12	11	0.5	180	0
GR3			13	11	0.5	240	<2
GR4			10	11	0.5	40	-
GR5 *			14	11	0.5	600	2.5
GR6			14	11	1	>1200	>3
GR7			10	11	2	210	0
GR8			12	11	2	-	1
GR9 *			12	11	1	>600	2.85
GR10 **			14	10	2	240	2.12
GR11			14	12	2	1200	>3
GR12			14	8	2	60	-
GR13			14	10	0	1200	>3
GR14			14	10	4	120	-
GR15			12	10	2	90	-
GR16			16	10	2	1200	>3

* Samples complying with the ISSA specification; ** Sample corresponding to the optimum component content selected within the scope of the study.

. Although GR5 and GR9 satisfied the initial limit conditions, they were not selected due to their inferior performance in the cohesion test compared to GR10 (results presented in the following sections). Specifically, visual inspection the following day revealed that the insufficient cohesion in these samples resulted in a friable structure. This observation highlighted the necessity for cement addition to ensure structural integrity. Consequently, the GR10 specimen, identified as the optimum mixture, contains a higher proportion of cement compared to the other candidates.

In composite aggregate mixtures utilizing both basalt and granite, particularly at a ratio of 50BA+50GR, an extremely short mixing time of approximately 10 s was observed, failing to meet the specification requirements despite significant increases in the liquid phase content (emulsion and water). Furthermore, even when the basalt fraction was reduced to 10% (with 90% GR), the mixing time did not exceed 80 s. This demonstrated that the combined use of basalt and granite aggregates is incompatible for slurry seal design due to premature breaking issues. Consequently, basalt–limestone combinations were evaluated as an alternative strategy to ensure the feasibility of utilizing basalt aggregates in slurry seal applications. Based on mixing time and consistency tests, it was determined that mixtures prepared with 70LS+30BA and 50LS+50BA ratios satisfied the limit values specified in the ISSA guidelines. For these compliant mixtures, the optimum component dosages were identified as follows: for the 50LS+50BA combination, 13% water, 18% emulsion, and 2% cement; whereas for the 70LS+30BA combination, these values were determined as 14% water, 15% emulsion, and 1% cement, as detailed in

Table 7. Determination of optimum slurry seal mixtures based on different aggregate proportions.

Sample No	Aggregate Type (%)			Distribution of Mixture Components (%)			Mixing Time (s)	Consistency (cm)
	Granite	Limestone	Basalt	Water	Emulsion	Cement
GRBA1	50	-	50	11	15	0	10	-
GRBA2	50	-	50	15	20	0	70	-
GRBA3	90	-	10	11	15	0	21	-
GRBA4	90	-	10	15	20	0	80	-
LSBA5	-	50	50	12	15	1	40	-
LSBA6	-	90	10	14	16	0	>420	-
LSBA7	-	70	30	13	15	0	160	-
LSBA8	-	70	30	13	15	2	220	2.00
LSBA9	-	60	40	13	15	2	120	-
LSBA10	-	70	30	13	15	1	190	1.25
LSBA11	-	70	30	15	15	1	-	3.75
LSBA12 **	-	70	30	14	15	1	235	2.75
LSBA13	-	70	30	14.5	15	1	-	3.25
LSBA14	-	40	60	14	15	2	50	-
LSBA15	-	40	60	14	20	2	210	<2.0
LSBA16	-	30	70	14	20	2	150	-
LSBA17	-	50	50	15	20	2	>420	>3.0
LSBA18	-	-	100	14	15	2	10	-
LSBA19	-	-	100	14	20	2	40	-
LSBA20	-	50	50	12	15	1	40	-
LSBA21	-	50	50	15	20	2	>420	>3.0
LSBA22	-	50	50	14	18	2	230	3.06
LSBA23	-	50	50	13	17	2	140	-
LSBA24 *	-	50	50	13	18	2	200	2.54

* Samples complying with the ISSA specification, ** Sample corresponding to the optimum component content selected within the scope of the study.

.

In this context, four distinct aggregate compositions (LS100, BA100, LS70+BA30, and LS50+BA50) meeting ISSA criteria at their estimated optimum emulsion contents were identified. To assess performance sensitivity, specimens were prepared at three emulsion levels Optimum-2%, Optimum%, and Optimum+2% derived from these reference values, and were subsequently subjected to the comprehensive laboratory testing regime prescribed by ISSA specifications.

3.2. Wet Track Abrasion Test (WTAT)

In this experiment, the Wet Track Abrasion Test (WTAT) was conducted in accordance with ISSA TB100 [62] and ASTM D3910 [58] standards to evaluate the abrasion resistance of bituminous slurry seal mixtures under wet conditions and the adhesion performance of the bitumen emulsion to the aggregate. To analyze the sensitivity of the mixtures to binder variations, the test protocol was applied not only to specimens prepared at the determined optimum emulsion content but also to those with emulsion rates of Optimum-2% and Optimum+2% for each mixture design. During specimen preparation, the appropriate mixture components (cement, water, and emulsion) were added to approximately 800 g of graded aggregate. The resulting slurry was cast into standard molds and cured in a forced-draft oven at 60 °C for a minimum of 15 h until a constant mass was achieved. Prior to testing, the cured specimens were conditioned in a water bath at 25 °C for a period of 60 to 75 min. The abrasion process was performed using a Hobart N-50 mixer (Figure 6), subjecting the submerged specimen surface to mechanical abrasion for a duration of 300 s (5 min). Upon completion of the test cycle, the specimens were washed to remove loose particles, dried to constant mass, and the resulting mass loss was calculated. The Wet Track Abrasion Test (WTAT) results obtained for the various aggregate combinations and emulsion contents (Optimum% and Optimum ± 2%) are presented in

Table 8. WTAT results of specimens containing Optimum%, Optimum+2%, and Optimum-2% emulsion prepared using different aggregate types.

Specimen ID	Initial Weight (g)	Final Weight (g)	Abrasion Loss (g)	Corrected Loss (g/m2)	ISSA Standard
LS100/OPT−2	563.7	558.0	5.7	166.73	<538 g/m2
LS100/OPT	659.9	657.3	2.6	76.05
LS100/OPT+2	628.8	626.0	2.8	81.90
GR100/OPT−2	546.9	527.4	19.5	569.63
GR100/OPT	619.7	614.4	5.3	155.03
GR100/OPT+2	634.1	629.4	4.7	137.48
LS70+BA30/OPT−2	513.3	505.2	8.1	236.92
LS70+BA30/OPT	558.1	551.3	6.8	197.68
LS70+BA30/OPT+2	563.6	562.4	1.2	35.10
LS50+BA50/OPT−2	512.5	504.5	8.0	234.00
LS50+BA50/OPT	589.3	585.2	4.1	119.92
LS50+BA50/OPT+2	564.0	560.6	3.4	99.45

Note: Specimen IDs were defined based on aggregate type and proportion, where LS, GR, and BA represent limestone, granite, and basalt aggregates, respectively, and the numbers indicate their percentage by weight. OPT denotes the optimum emulsion content, while OPT−2 and OPT+2 indicate emulsion contents 2% below and above the optimum value.

below.

As shown in Figure 7, the results of the Wet Track Abrasion Test (WTAT) revealed a significant reduction in Abrasion Loss (AL) values with increasing emulsion content in bituminous slurry seal mixtures prepared with solely limestone and solely granite aggregates. At low emulsion contents, insufficient coating of aggregate surfaces by the binder leads to the detachment of aggregates from the specimen matrix under abrasive forces, thereby resulting in increased material loss. The highest AL value was recorded in the mixture containing solely granite aggregate at the Optimum−2% emulsion rate; this value exceeded the threshold of 538 g/m² specified in the ISSA standard. A similar trend was observed in mixtures containing varying proportions of limestone and basalt, where AL values decreased as the emulsion content increased. Among the limestone–basalt combinations, the maximum abrasion loss was identified in the mixture containing LS70+BA30 aggregate at the Optimum−2% emulsion rate; however, this value remained below the limit specified by the ISSA standard.

3.3. Cohesion Test

The cohesion test [63] was conducted to evaluate the time-dependent setting and curing characteristics of the bituminous slurry seal mixtures. In the experimental procedure, approximately 400 g of graded aggregate was utilized. Cement, water, and bitumen emulsion were added to the aggregate at the optimum proportions previously determined through mixing time and consistency tests. The mixture preparation protocol was consistent with the standard procedures applied in the other tests.

Following the mixture preparation, five circular metal molds were employed to facilitate torque measurements at various curing intervals. The fresh mixture, prepared with optimum component contents, was immediately poured into molds positioned on bitumen-saturated roofing felt squares. The surface of the specimen within the mold was leveled using a spatula to ensure a smooth finish. The simultaneously prepared specimens were allowed to cure for predetermined intervals prior to testing.

Before testing, each specimen was demolded and placed into the cohesion tester along with the roofing felt base. The cohesion testing apparatus and the representative placement of a specimen are illustrated in Figure 8. During the test, the resistance of the specimen to rotational motion was measured under a pressure of 2 bar (200 kPa), and the resulting torque values were recorded in kg-cm. Measurements were taken at 15, 30, 60, 90, and 120 min. Additionally, a 24 h measurement was recorded to evaluate the long-term curing behavior.

The cohesion test results for the mixtures prepared with different aggregate types at their optimum emulsion contents are presented in Figure 9. In the cohesion test, the time required for the torque value to reach a minimum of 12 kg-cm is defined as the set time, while the time required to reach a minimum of 20 kg-cm is defined as the cure time [32]. The torque values of different mixtures at various curing times are presented in

Table 9. Torque values (kg-cm) of different aggregate mixtures at various curing times.

Curing Time (min)	LS100 (kg-cm)	GR100 (kg-cm)	LS70+BA30 (kg-cm)	LS50+BA50 (kg-cm)
15	9	8	8	8
30	8	9	9	9
60	11	10	11.5	13
90	13	18	13	16
120	13	16	14	20
1440	20	24.8	24	24

. It was determined that the set time occurred within the 60–90 min range for mixtures prepared using solely limestone and solely granite aggregates. For both mixtures, the cure time—indicated by the torque value reaching 20 kg-cm—was achieved at the end of 24 h.

When evaluating the limestone–basalt mixtures, it was observed that the set time for the LS70+BA30 mixture fell within the 60–90 min range, whereas for the LS50+BA50 mixture, it occurred within the 30–60 min range. This indicates that the LS50+BA50 mixture exhibits an earlier setting time. Regarding the cure times, it was determined that the cure time for the LS50+BA50 mixture was completed between 90 and 120 min, while curing for the LS70+BA30 mixture was achieved at the end of 24 h. These results demonstrate that increasing the basalt proportion has an accelerating effect on the setting and curing behavior of the mixture.

3.4. Loaded Wheel Test (LWT) (Sand Adhesion)

The Loaded Wheel Test (LWT) [64] was conducted to evaluate excess bitumen content in bituminous slurry seal mixtures, based on the principle of quantifying sand adhesion on a specimen subjected to wheel tracking. The procedural stages are illustrated in Figure 10. In accordance with the standard recommendation (mold depth >25% of maximum aggregate size), a 3 mm deep mold was selected for the Type 1 gradation (maximum aggregate size: 2.36 mm). The mixture was prepared using approximately 400 g of aggregate following the standard protocol, cast into the mold on a metal plate, and leveled. Specimens were cured in an oven at 60 °C until constant mass was achieved (∼18 h).

Following the curing process, the specimens were subjected to 1000 compaction cycles in the LWT apparatus under a constant load of 56.7 kg at a rate of 44 cycles/min. Subsequently, the specimens were washed to remove loose debris, dried at 60 °C for 30 min, cooled, and weighed (IW). To determine sand adhesion, 300 g of fine sand (No. 30–No. 100) heated to 82.2 °C was evenly spread over the specimen using a frame, and the apparatus was operated for an additional 100 cycles. Unadhered sand was removed, and the final weight (FW) was recorded. Sand adhesion was calculated as the difference between FW and IW (g/m²). The LWT results for mixtures containing solely limestone, solely granite, and limestone–basalt combinations at varying emulsion contents (Optimum% and ±2%) are presented in

Table 10. LWT results of specimens containing Optimum%, Optimum+2%, and Optimum−2% emulsion prepared using different aggregate types.

Specimen       ID	Initial Weight (g)	Final Weight (g)	Sand Adhesion (g)	Sand Adhesion (g/m2)	ISSA Standard
LS100/OPT−2	240.5	241.3	0.8	88.59	<538 g/m2
LS100/OPT	248.9	250.1	1.2	132.89
LS100/OPT+2	245.4	246.9	1.5	166.11
GR100/OPT−2	253.5	253.7	0.2	22.15
GR100/OPT	236.8	238.0	1.2	132.89
GR100/OPT+2	260.2	261.7	1.5	166.11
LS70+BA30/OPT−2	237.1	238.7	1.6	177.19
LS70+BA30/OPT	249.5	251.3	1.8	199.34
LS70+BA30/OPT+2	238.3	239.5	1.2	132.89
LS50+BA50/OPT−2	253.4	254.6	1.2	132.89
LS50+BA50/OPT	245.4	246.9	1.5	166.11
LS50+BA50/OPT+2	233.1	235.1	2.0	221.48

Note: Specimen IDs were defined based on aggregate type and proportion, where LS, GR, and BA represent limestone, granite, and basalt aggregates, respectively, and the numbers indicate their percentage by weight.

.

An examination of the Loaded Wheel Test (LWT) results presented in Figure 11 reveals that sand adhesion values increased with increasing emulsion content. All obtained results were observed to remain within the specification limits defined by the ISSA standards. Regarding aggregate composition, it was determined that sand adhesion values were higher in mixtures where the limestone proportion was dominant compared to basalt. This variation is attributed to the chemical and mineralogical structure of the basalt, which directly influences the performance characteristics of the mixture.

3.5. Determination of Optimum Bitumen Emulsion Content

The primary objective of this study is to determine the variations in bitumen emulsion demand of slurry seal mixtures prepared with different aggregate types, specifically limestone (LS), granite (GR), and basalt (BA), and to establish optimum performance criteria. While limestone and granite aggregates generally exhibited behavior consistent with standard design protocols, the use of 100% basalt presented significant challenges. Consistent with current literature, it was observed that basalt aggregate exhibits high specific surface area and surface roughness, a condition that necessitates a more sensitive application procedure [13]. Furthermore, the chemical composition of the basalt aggregate adversely affected mixability properties, causing difficulties in meeting the mixing time and consistency requirements specified in ISSA standards. In preliminary trials, it was determined that mixtures containing 100% basalt failed to meet the desired criteria even at the lower and upper limits of component usage proportions.

To overcome these workability and consistency problems, a ’partial replacement’ strategy was adopted. Basalt aggregate was modified by substituting it with limestone at rates of 30% and 50% (LS70+BA30 and LS50+BA50 mixtures, respectively). This modification successfully optimized the mix design, allowing the mixtures to satisfy the necessary mixing time and consistency standards.

Following consistency verification, the optimum emulsion content (OPT) was determined. To evaluate the sensitivity of the mixtures, new specimens were produced by varying the emulsion content by ±2% by weight from the theoretical optimum point (OPT−2, OPT, OPT+2). The optimum binder content was determined by graphically analyzing the intersections and trends of the Wet Track Abrasion Test (WTAT) and Loaded Wheel Test (LWT) results.

Upon examining the graphical analyses presented in Figure 12, the degrees of correspondence between the theoretical design values and experimental performances of the pure (LS100, GR100) and substituted (LS70+BA30, LS50+BA50) mixtures are clearly observed. In this context, the LS70+BA30 mixture, in particular, exhibited a remarkable performance, achieving an ’Exact Match’ at the optimum point. In the aforementioned mixture, the flawless intersection of the WTAT (abrasion loss) and LWT (bleeding/adhesion risk) curves at the determined optimum emulsion content demonstrates that a highly balanced mix design was obtained. When the other mixtures are compared, the GR100 specimen showed a strong agreement of approximately 86.9% between the theoretical and experimental optimums, whereas, in the LS50+BA50 mixture, this convergence rate remained at the level of 66.7%. This finding indicates that although the 50% basalt replacement improved workability compared to the use of 100% basalt, the sensitivity of the mixture to emulsion changes is still higher than that of the LS70+BA30 combination.

The varying breaking behaviors of slurry seal mixtures result from the bitumen emulsion–aggregate interaction, which is governed by the acidity–alkalinity balance, surface charge characteristics, and microscopic morphology of the aggregates. The rate of this interaction is influenced by the electrostatic attractions and chemical reactions between them. In other words, the oxide compositions significantly differ among various aggregate types, and specific oxide ratios are reported to have a substantial impact on the breaking rate [65]. The rapid breaking caused by basalt in this study can be explained by the aggregate’s high surface energy (reactive) and rough structure, the non-homogeneous distribution of bitumen emulsion droplets on the aggregate surface, and the sudden breaking behavior resulting from irregular electrostatic attraction between the positively charged cationic emulsion and local negative charges on the basalt surface (oxides such as ${\mathrm{Fe}}_2{\mathrm{O}}_3$, $\mathrm{MgO}$, $\mathrm{CaO}$, and ${\mathrm{Al}}_2{\mathrm{O}}_3$ forming local basic sites upon contact with water). However, considering the combination of basalt and limestone, the less rough surface morphology and the, ${\mathrm{Ca}}^{2+}$ content in the mineralogical structure of limestone provided a more controlled electrostatic attraction with the cationic emulsion, allowing sufficient time to achieve a homogeneous mixture [52]. Consequently, the LS70+BA30 mixture was identified as the most suitable experimental output for optimum conditions, benefiting from the chemical compatibility of limestone for workability and the mechanical strength of basalt for performance.

4. Statistical Analysis

4.1. Analysis of Variance (ANOVA)

This study is based on a comprehensive experimental dataset established to investigate the effects of aggregates with different mineralogical origins (granite, limestone, and basalt) and varying proportions of liquid (emulsion, water) and solid (cement) components on the fresh properties of bituminous mixtures, specifically mixing time and consistency values. Analysis of Variance (ANOVA) and correlation analyses were employed as the primary evaluation tools to determine the individual and interactive effects of mix design parameters on rheological performance, to assess significance levels, and to reveal the direction of linear relationships among the variables.

In the analysis process, censored data falling outside measurement limits (e.g., “>300”) and missing observations resulting from reactions too rapid to measure (“-”) were incorporated into the analysis by assigning physically consistent lower and upper boundary values (5 s for rapid set, 0 cm for zero flow) to ensure the statistical processability of the dataset and maintain analysis integrity. The General Linear Model (GLM)-based Analysis of Variance (ANOVA) results, conducted under this methodological assumption and presented in

Table 11. ANOVA results summary.

Source	Sum of Sq	df	Mean Square	F	p-Value	Partial Eta Sq
Cohesion
Time	637.00	5	127.40	61.303	0.000	0.953
Mix Type	21.84	3	7.28	3.504	0.042	0.412
Error	31.17	15	2.08	-	-	-
LWT
Mix Type	27,595.15	3	9198.38	15.003	0.003	0.882
Ratio Level	24,608.39	2	12,304.20	20.068	0.002	0.870
Error	3678.71	6	613.12	-	-	-
WTAT
Mix Type	75,317.97	3	25,105.99	24.364	0.001	0.924
Ratio Level	3567.89	2	1783.95	1.731	0.255	0.366
Error	6182.72	6	1030.45	-	-	-
Mix Time
Emulsion	374,400.94	1	374,400.94	4.095	0.049	0.079
Water	447,548.61	1	447,548.61	4.895	0.032	0.093
Cement	67,283.40	1	67,283.40	0.736	0.395	0.015
Aggregate Type	2,014,537.50	2	1,007,268.75	11.017	0.000	0.315
Error	4,388,460.10	48	91,426.25	1.000	0.500	-
Consistency
Emulsion	7.59	1	7.59	4.707	0.035	0.089
Water	15.88	1	15.88	9.850	0.003	0.170
Cement	0.41	1	0.41	0.255	0.616	0.005
Aggregate Type	30.51	2	15.26	9.461	0.000	0.283
Error	77.40	48	1.61	1.000	0.500	-

, reveal that aggregate origin is the most determinant factor on mixing time. The obtained statistical data demonstrate that the effect of aggregate type on mixing time is highly significant at the 99% confidence level ($F=11.017$, $p<0.001$) and accounts for a substantial portion of the total variance (${\eta }_p^2=0.315$) on its own. Furthermore, while the amounts of emulsion and water constituting the liquid phase were found to be statistically significant ($p<0.05$) for mixing time, they were determined to play a secondary role compared to aggregate type in terms of effect size (${\eta }_p^2=0.079$ and $0.093$, respectively).

When the consistency values, which serve as an indicator of rheological behavior, were examined, a shift in the hierarchy of influence among the mix design parameters was observed. While the significant effect ($p<0.001$) of aggregate type—stemming from its physical constraints—persisted in this parameter, the determining role of water content on the system’s fluidity increased markedly. The partial eta squared value of water content for consistency (${\eta }_p^2=0.170$) nearly doubled compared to its impact on mixing time, confirming the dominant role of water ($F=9.850$, $p=0.003$) on consistency. In contrast, it was understood that the effect of variations in cement content on mixing time ($p=0.395$) and consistency values ($p=0.616$) remained much more limited compared to other components and did not reach statistical significance; thus, this parameter played a negligible (minor) role in the system rheology alongside the dominant factors.

4.2. Pearson Correlation Coefficients

Upon examining the obtained correlation matrix results, it is observed that the characteristics of the relationships established by each aggregate group with the mixture components differ significantly from one another. This situation is of critical importance for understanding intra-group dynamics that might be overlooked in the general Analysis of Variance (ANOVA).

In this context, the Pearson Product-Moment Correlation Coefficient (r) was utilized to quantify the direction and strength of the linear relationship between mixture parameters (independent variables) and rheological properties (dependent variables). This coefficient, which is a parametric analysis method based on the assumption that the data follows a normal distribution, is calculated as shown in Equation (1), based on the principle of the ratio of the covariance between two variables to the product of their standard deviations:

$$r={\displaystyle \frac{{\sum }_{i=1}^n(X_i-\overline{X})(Y_i-\overline{Y})}{\sqrt{{\sum }_{i=1}^n{(X_i-\overline{X})}^2}\sqrt{{\sum }_{i=1}^n{(Y_i-\overline{Y})}^2}}}$$

(1)

where n represents the sample size, $X_i$ and $Y_i$ denote the individual observation values, and $\overline{X}$ and $\overline{Y}$ denote the arithmetic means of the respective variables. The calculated r value theoretically ranges between $-1$ and $+1$. A positive coefficient ($r>0$) indicates that as one variable increases, the other also increases (e.g., consistency increases with increasing water content), whereas a negative coefficient ($r<0$) signifies an inverse relationship (e.g., fluidity decreases with increasing cement content). The proximity of the coefficient to 1 in absolute value indicates the strength of the linear dependence between the variables.

The obtained Pearson correlation coefficients (

Table 12. Pearson correlation coefficients for mixing time and consistency (limestone and granite only).

	Mixing Time		Consistency
Parameter	Limestone	Granite	Limestone	Granite
Emulsion	0.186	0.544	0.524	0.647
Water	0.625	0.236	0.571	0.319
Cement	0.081	−0.226	−0.140	−0.369

) demonstrate that the response of homogeneous aggregate groups to liquid phase components differs significantly depending on their mineralogical origin. In the limestone group, it is observed that water is the parameter dominating the system, establishing the strongest positive relationship with both mixing time ($r=0.625$) and consistency ($r=0.571$). In contrast, in the granite group, the control of rheological behavior shifts to the emulsion content. While emulsion exhibits the highest correlation with mixing time ($r=0.544$) and consistency values ($r=0.647$) in granite specimens, the consistency-reducing (negative) effect of cement ($r=-0.369$) in this group is more pronounced compared to the limestone group.

In the heterogeneous mixture group containing different proportions of basalt (

Table 13. Pearson correlation coefficients for basalt-based mixtures.

Parameter	Mixing Time	Consistency
Basalt Content	−0.154	−0.129
Emulsion	0.569	0.276
Water	0.393	0.666
Cement	0.081	0.220

), the mechanism of influence of the parameters diverges. In this group, the primary factor prolonging the mixing time ($T_{mix}$) became the emulsion ($r=0.569$), surpassing the effect of water; this indicates that the retarding or process-prolonging effect of the emulsion increases in basalt-based systems. Conversely, it was determined that water is the main determinant defining the workability (consistency) of the mixture with a very strong correlation ($r=0.666$). The basalt content included in the model exhibited a negative but weak relationship with both mixing time ($r=-0.154$) and consistency ($r=-0.129$); these data indicate that the increase in basalt amount constrains the rheological values to a limited extent.

When a general assessment is made, it is understood that aggregate surface properties and absorption capacities are directly reflected in the correlation matrix. While limestone-based mixtures display a more water-sensitive behavioral model rheologically, the lubricating effect of the emulsion becomes dominant in acidic-character granite aggregates. In basalt mixtures, however, an interesting dichotomy emerged; it was observed that the emulsion controls the chemical reaction kinetics (time), while water distinguishes itself as the main parameter managing physical fluidity (consistency). These findings offer a statistical basis regarding which liquid component should be optimized depending on the aggregate type in mixture design.

In light of these findings, it was concluded that aggregate-specific engineering design strategies can be defined for both laboratory and field applications. In this context, for the design of limestone-based mixtures, priority should be given to optimizing the water-to-binder ratio due to the dominant influence of water content. For granite-based mixtures, on the other hand, the importance of considering emulsion content as the primary design parameter is highlighted. In contrast, for basalt-based hybrid mixtures, it was determined that adopting a dual optimization strategy is appropriate, taking into account both the effect of emulsion dosage on chemical reactions and the role of water in governing physical fluidity.

5. Conclusions

Within the scope of this study, the use of aggregates with different mineralogical origins in slurry seal mixtures was comprehensively investigated in terms of performance, durability, and component interactions. Based on the experimental data and statistical analyses, the main findings are summarized as follows:

• The LS70+BA30 hybrid mixture reflected the synergistic effect of different aggregates and achieved a complete match between the theoretical design and experimentally determined optimum points. The high surface reactivity of basalt and the low surface reactivity of limestone provided an ideal balance between workability and early strength.
• WTAT tests revealed that pure granite mixtures exceeded specification limits and performed inadequately at low emulsion contents, whereas basalt-substituted mixtures maintained structural integrity across all emulsion ratios.
• According to the LWT results, the LS50+BA50 combination, which reached a torque value of 20 kg-cm at 120 min, was identified as the most suitable alternative for projects requiring rapid opening to traffic.
• ANOVA statistical analysis results demonstrated that aggregate origin was the primary factor influencing mixing time ($F=11.017$), while water content was the dominant factor affecting consistency ($F=9.850$).
• Correlation matrices indicated that mixtures containing limestone aggregates were more sensitive to variations in water content, whereas granite-based mixtures were more sensitive to changes in emulsion content. In basalt-containing hybrid mixtures, emulsion content governed mixing time, while water content controlled consistency.

These findings present an innovative hybrid mixture proportion that can be utilized in engineering design for material selection, determination of component ratios, and development of optimum performance solutions. Although the present study focused on evaluations based on physical and rheological parameters, future research aims to validate the field performance of the proposed hybrid mixtures under real traffic loading conditions and to investigate, at the field scale, the long-term durability effects of environmental factors such as freeze–thaw cycles and UV aging, as well as to conduct detailed microscopic-level analyses.